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[[Image:Am{{Short addon manager.pngdescription|right|thumb|AddPlug-on Manager from Windowsin XPmodule SP2for Internet Explorer]]}}
[[File:Am addon manager.png|right|thumb|Add-on Manager from [[Windows XP]] SP2 Internet Explorer]]
A '''Browser Helper Object''' ('''BHO''') is a [[Library (computer science)|DLL]] [[Module (programming)|module]] designed as a [[Plug-in (computing)|plugin]] for the [[Microsoft]]'s [[Internet Explorer]] [[web browser]] to provide added functionality. BHOs were introduced in October 1997 with the release of [[Internet Explorer 4|version 4]] of Internet Explorer. Most BHOs are loaded once by each new instance of Internet Explorer. However, in the case of [[Windows Explorer]], a new instance is launched for each window.
 
BHOs are still supported as of Windows 10, through [[Internet Explorer 11]], while BHOs are not supported in [[Microsoft Edge]].
== Implementation ==
Each time a new instance of Internet Explorer starts, it checks the [[Windows Registry|windows registry]] for the following key:<br />
''HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Browser Helper Objects''<br />
If Internet Explorer finds this key in the registry, it looks for a [[CLSID]] key listed below the key. The CLSID keys under Browser Helper Objects tell the browser which BHOs to load. Removing the registry key prevents the BHO from being loaded.<br />
For each CLSID that is listed below the BHO key, Internet Explorer calls CoCreateInstance to start the instance of the BHO in the same process space as the browser. If the BHO is started and implements the IObjectWithSite interface , it can control and receive events from Internet Explorer. BHOs can be created in any language that supports [[Component Object Model|COM]].<ref>Roberts Scott, ''Programming Microsoft Internet Explorer 5'', Microsoft Press, 1999, ISBN 0-7356-0781-8</ref>▼
 
== Examples of BHOImplementation ==
▲Each time a new instance of Internet Explorer starts, it checks the [[Windows Registry]] for the key ''HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Browser Helper Objects''. If Internet Explorer finds this key in the registry, it looks for a [[Universally unique identifier#In COM|CLSID]] key listed below the key. The CLSID keys under Browser Helper Objects tell the browser which BHOs to load. Removing the registry key prevents the BHO from being loaded. For each CLSID that is listed below the BHO key, Internet Explorer calls CoCreateInstance to start the instance of the BHO in the same process space as the browser. If the BHO is started and implements the IObjectWithSite interface , it can control and receive events from Internet Explorer. BHOs can be created in any language that supports [[Component Object Model|COM]].<ref>Roberts Scott, ''Programming Microsoft Internet Explorer 5'', Microsoft Press, 1999, {{ISBN |0-7356-0781-8}}</ref>
 
== Examples ==
Some modules enable the display of different file formats not ordinarily interpretable by the browser. The [[Adobe Acrobat]] plug-in that allows Internet Explorer users to read [[Portable Document Format|PDF]] files within their browser is a BHO.
 
Other modules add toolbars to Internet Explorer, such as the [[Alexa Toolbar]] that provides a list of web sites related to the one you are currently browsing, or the [[Google Toolbar]] that adds a toolbar with a Google search box to the browser [[user interface]].
 
The Conduit toolbars are based on a BHO that can be used on [[Internet Explorer 7]] and up. This BHO provides a search facility that connects to [[Microsoft]]'s [[Bing (search engine)|Bing]] search.
 
== Concerns ==
The BHO [[application programming interface|API]] exposes [[hooking|hook]]s that allow the BHO to access the [[Document Object Model]] (DOM) of the current page and to control navigation. Because BHOs have unrestricted access to the Internet Explorer event model, some forms of [[malware]] (such as adware and spyware) have also been created as BHOs.<ref>{{Cite web|title=Browser Hijack Objects (BHOs)|url=https://blog.malwarebytes.com/threats/browser-hijack-objects-bhos/|access-date=2021-12-05|website=Malwarebytes Labs|language=en-US}}</ref><ref>{{Cite journal |last=Park |first=Beomsoo |last2=Hong |first2=Sungjin |last3=Oh |first3=Jaewook |last4=Lee |first4=Heejo |date=2005 |editor-last=Kantor |editor-first=Paul |editor2-last=Muresan |editor2-first=Gheorghe |editor3-last=Roberts |editor3-first=Fred |editor4-last=Zeng |editor4-first=Daniel D. |editor5-last=Wang |editor5-first=Fei-Yue |editor6-last=Chen |editor6-first=Hsinchun |editor7-last=Merkle |editor7-first=Ralph C. |title=Defending a Web Browser Against Spying with Browser Helper Objects |url=https://link.springer.com/chapter/10.1007/11427995_85 |journal=Intelligence and Security Informatics |series=Lecture Notes in Computer Science |language=en |location=Berlin, Heidelberg |publisher=Springer |pages=638–639 |doi=10.1007/11427995_85 |isbn=978-3-540-32063-0}}</ref>
 
The BHO [[application programming interface|API]] exposes [[hooking|hook]]s that allow the BHO to access the [[Document Object Model]] (DOM) of the current page and to control navigation. Because BHOs have unrestricted access to the Internet Explorer event model, some forms of [[malware]] have also been created as BHOs. For example, the [[Download.ject]] malware installsis a BHO that wouldis activateactivated upon detectingwhen a secure [[HTTP]] connection is made to a financial institution, then begins to [[keystroke logging|record keystrokes]] for the purpose of capturing user's keystrokespasswords. The [[MyWay Searchbar]] (intendingtracks tousers' capturebrowsing passwords)patterns and transmitpasses the information toit arecords websiteto usedthird byparties. RussianThe [[computer crime|computer criminalsC2.LOP]]. malware Otheradds BHOslinks suchand aspopups theof [[MyWayits Searchbar]]own trackto users'web browsingpages patternsin andorder passto thedrive information they recordusers to third[[pay-per-click]] partieswebsites.{{cn|date=May 2021}}
 
Many BHOs introduce visible changes to a browser's interface, such as installing toolbars in [[Internet Explorer]] and the like, but others run without any change to the interface. This renders it easy for malicious coders to conceal the actions of their browser add-on, especially since, after being installed, the BHO seldom requires permission before performing further actions. For instance, variants of the ClSpring trojan use BHOs to install scripts to provide a number of instructions to be performed such as adding and deleting registry values and downloading additional executable files, all completely transparently to the user.<ref>Computer Associates malware entry at [http://www.ca.com/us/securityadvisor/virusinfo/virus.aspx?id=42280 ca.com], retrieved 1/16/2009</ref> The [[DyFuCA]] spyware even replaces Internet Explorer's general error page with an ad page.
 
In response to the problems associated with BHOs and similar extensions to Internet Explorer, Microsoft debuted an ''Add-on Manager'' in [[Internet Explorer 6]] with the release of [[Windows XP#Service Pack 2|Service Pack 2]] for [[Windows XP]] (updating it to IE6 Security Version 1, a.k.a. SP2). This utility displays a list of all installed BHOs, [[browser extension]]s and [[ActiveX control]]s, and allows the user to enable or disable them at will. There are also free tools (such as BHODemon) that list installed BHOs and allow the user to disable malicious extensions. [[Spybot – Search & Destroy|Spybot S&D]] advanced mode has a similar tool built in to allow the user to disable installed BHOsBHO.
 
== See also ==
*[[Browser extension]]
*[[Plug-in (computing)]]
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== External links ==
*[https://sites.google.com/site/bhosearch/ Sites.google.com] {{Webarchive|url=https://web.archive.org/web/20141224114851/https://sites.google.com/site/bhosearch/ |date=2014-12-24 }}
 
=== Microsoft sites ===
*[http://support.microsoft.com/default.aspx?scid=kb%3BEN-US%3Bq179230 ''IEHelper-Attaching to Internet Explorer 4.0 by Using a Browser Helper Object'']
*[https://web.archive.org/web/20050109091250/http://www.microsoft.com/windowsxp/using/web/sp2_addonmanager.mspx Control Internet Explorer Add-ons with Add-on Manager] - {{snd}}an article on Microsoft.com that explains this new feature of Windows XP Service Pack 2
*[http://msdn2.microsoft.com/en-us/library/bb250489.aspx Building Browser Helper Objects with Visual Studio 2005] - {{snd}}an October 2006 MSDN article by Tony Schreiner and John Sudds
 
=== Listings and examples ===
*[http://www.systemlookup.com/lists.php?list=1 CLSID List] - {{snd}}master list created by Tony Kleinkramer, which attempts to record and identify every BHO available (previously located at - {{snd}}the now defunct - {{snd}}[[CastleCops|castlecops.com]]) - {{snd}}also includes Toolbar, Explorer Bar and URLSearchHook GUIDs
*[http://www.adp-gmbh.ch/win/com/bho.html C++ example code for a BHO]
*[http://www.codeproject.com/Articles/19971/How-to-attach-to-Browser-Helper-Object-BHO-with-C C# example code for a BHO]
 
{{Information security}}
{{Microsoft APIs}}
{{Internet Explorer}}
{{Web interfaces}}
 
[[Category:Internet Explorer]]
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